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Importance of PTR Records for Reliable Mail Delivery

by Christopher on September 25, 2011

Outgoing email rejections are becoming increasingly common, and while this is good news in the fight against spam, it can hurt your business if your emails are not reaching their intended targets. One reason for the increase in rejections is that growing numbers of incoming mail servers are rejecting emails from outgoing servers without a valid PTR record.

A PTR record, or pointer record, enables someone to perform a reverse DNS lookup. This allows them to determine your domain name based on your IP address. Because generic domain names without a PTR are often associated with spammers, incoming mail servers identify email from hosts without PTR records as spam. If you do not have a PTR, these same servers may identify legitimate emails from your business as spam and block them from reaching your customers.

Why Are PTR Records Important?

There is no requirement that you have a valid PTR record, but you may be unable to successfully send email to certain sites if you do not.

PTR records are most commonly used by spam filters to determine the suspiciousness of an email. Because spammers are more likely to use fake domain names and dial-up IP addresses, they are considerably less likely than legitimate organizations to have a valid PTR record. If a spam filter determines that your PTR record is generic or invalid, it is more likely to classify your email as junk mail. Blocked emails can result in lost business, damaged relationships with clients, and wasted time.

Unless you are certain that you have a valid PTR, do not assume your business is not at risk simply because you’ve received few complaints about missing emails; you may still have a problem. Because many organizations do not have matching PTR records, including those that use dynamic IP addresses, many spam filters use this criteria as part of a weighted algorithm in an attempt to reduce false-positives. Therefore, some or most of your emails may be getting through, leading you to erroneously believe your company is not being negatively affected. Unfortunately, it may take only one blocked email to seriously harm your organization.

In some cases, your email will be summarily rejected if you do not have a valid PTR record. AOL, for example, rejects email messages without valid PTRs. To avoid triggering spam filters, your PTR record should match with the host name on port 25 when the receiving server returns a verification check.

Contact your ISP to verify you have a valid PTR record or to request they create a PTR for your mail server IP address. PTR records are not created when you register your domain name; it is the responsibility of your ISP to create them. If your business uses a dynamic address and you cannot obtain a PTR, you can work around the problem by sending your outgoing mail through a server that does have a valid PTR record.

Ideally, you should stick to one PTR unless your business has a specific need for more than one. While DNS does not restrict the number of PTR records associated with each reverse DNS entry, having multiple records for a single IP address is generally a bad idea.

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